Well look at the datasheet for the MPU6050, which will tell you how many I2C addresses you can have.

And if your RTC uses the same I2C address as the MPU6050, get a different brand of RTC which uses a different address. It is also possible, at least in principle, to have two separate I2C links, so that the address conflict between the slave devices is not an issue.

Some of the fancier microcontrollers already have this, I don't know how difficult it would be to do with the arduino, you would probably have to hack the Wire library somehow.